<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
  
</head>
<body>
  <div id="app">
    <button @click="addLi">添加li</button>
    <ul id="oul">
      <li v-for="item in arr" :key="item">
        {{ item }}
      </li>
    </ul>
  </div>
  <script src="./vue.js"></script>
  <script>
    const vm = new Vue({
      el: '#app',
      data: {
        arr: [1, 2, 3, 4]
      },
      methods: {
        addLi(){
          this.arr.push(this.arr.length+1);
          
          const oul = document.querySelector('#oul');
        
          // 观察者 什么时候触发 每一次数据更新 dom更新完成后触发
          this.$nextTick(() => {
            console.log(oul.children);
            oul.lastElementChild.style.background = 'red';
          })
        }
      }
    })
  </script>
</body>
</html>